There are a number of chemical reaction processes in which a relatively cooler stream of liquid is introduced into a relatively warmer solution. One of the concerns relating to such processes is the precipitation of solute from the warmer solution. One way to minimize this problem is to provide for rapid mixing of the solutions typically using some type of high intensity shear device such as a paddle or agitator stirrer. Generally, as the concentration of solute increases the solution often becomes more viscous and/or non-Newtonian and the rapid mixing of the relatively cooler solution and warm solution becomes more difficult. The problem is accentuated if the residence time in the reactor is relatively short. Further difficulties arise if the solute is difficult to re-dissolve in the solvent. This may lead to the formation of precipitate within the reactor which may ultimately affect the product. This problem is particularly acute where the process is constrained by heat or enthalpy transfer considerations.
All of the above issues are particularly relevant to bulk, mass, and solution polymerizations (as opposed to emulsion and suspension in which the diluent is usually water and heat of reaction is not a significant problem) in which there is a need to manage the heat of polymerization from a reactor. For some polymerizations this has led to the use of "chains" of reactors with the reactants being heated to successively higher temperatures and successively higher conversion in different reactors. In general, if the residence time in a reactor is relatively long (e.g. in the order of hours) and where the mixing time is relatively short (e.g. in the order of tens of minutes) there may not be too significant a problem.
In the continuous solution polymerization of olefins there are several problems. The residence time in the reactor is typically short and the lifetime of the catalyst at higher temperatures is also relatively short. Accordingly it is necessary to thoroughly, and quickly, mix the bulk reactor contents with the catalyst and reactor feed streams. After the catalyst is heated to the operating temperature of the reactor it has a short half life. The situation becomes worse where the viscosity of the solution rises (most notably when a high concentration of polymer is employed or when cooler conditions are used to make higher molecular weight polymer).
There have been several approaches to this problem. One approach has been to use tubular reactors. The high surface area of tube or loop reactors assist in the removal of heat of reaction. In order to avoid problems of precipitation, the reactor feed streams should be at temperatures above the precipitation temperature of the polymer from the solvent. However higher reactor temperature may also lead to the undesirable formation of low molecular weight polymer. Thus there are usually temperature limitations which restrict the operating flexibility of a tube or loop reactor.
U.S. Pat. No. 4,282,339 issued Aug. 4, 1981, assigned to National Distillers and Chemical Corp., teaches a process for the high pressure polymerization of alpha olefins in which dual autoclaves are used in tandem. The first reactor is a relatively higher pressure reactor (e.g. 30,000 psi). The product from the first reactor is cooled while still under high pressure and then introduced into a second reactor at a relatively lower pressure (e.g. 22,000 psi) and the polymerization is finished. The reference does not teach medium pressure polymerizations or suggest the type of mixing element of the present invention.
U.S. Pat. No. 4,496,698, issued Jan. 29, 1985, assigned to The Dow Chemical Company, takes a similar approach to the high pressure polymerization of ethylene in which the first reactor is operated at pressures of greater than 50,000 kilo Pascals ("kPa") (about 7,500 psi) and then the polymer melt is cooled and fed through a cooling heat exchanger to a second reactor which may be a tube or loop reactor. The reference does not teach medium pressure polymerization or suggest the type of mixing element of the present invention.
The paper Circulation Time Prediction in the Scale-up of Polymerization Reactors with Helical ribbon Agitators by D. F. Ryan, L. P. B. M. Janssen, and L. L. van Dierendonck, Chemical Engineering Science, Vol. 43, No. 8, pp.1961-1966, 1988 illustrates a chemical reactor (which may be used for polymerization) having a helical ribbon agitator but does not suggest a mixing element in accordance with the present invention.
The present invention seeks to provide a mixing element useful for rapid mixing of relatively cooler and warmer solutions, preferably in which the solvent is a hydrocarbon, to reduce the potential of solute precipitation.